A. Field of the Invention
The present invention relates to relay contact protective circuits and, in particular, to fail-safe arc suppression control circuits that can protect relay contacts.
B. Description of the Related Art
A relay is used to perform automatic electrical switching. A relay typically comprises two parts: a coil and a magnetic switch. When an electrical current flows through the coil, a magnetic field is created in proportion to the current flow through the coil. At a predetermined point, the magnetic field is sufficiently strong to pull the switch""s movable contact from its rest, or de-energized position, to its actuated, or energized position pressed against the switch""s stationary contact. When the electrical power applied to the coil drops, the strength of the magnetic field drops, releasing the movable contact and allowing it to return to its original de-energized position. As the contacts of a relay are opened or closed, there is an electrical discharge called arcing, which may cause heating and burning of the contacts and typically results in degradation and eventual destruction of the contacts over time.
U.S. Pat. No. 3,912,941 to Passarella and U.S. Pat. No. 4,250,531 to Ahrens both teach an arc prevention circuit for relays that switch inductive loads. These circuits have the relay contacts drive a transistor amplifier which drives the load, thereby eliminating contact arcing induced by inductive voltage transients. But contact arcing can still occur. Also, the transistor amplifier, which is not a perfect switch, must be designed to dissipate heat, and it is heated the entire time the load is supplied with current. Thus, a large power transistor with elaborate heat sinking may have to be provided. Except in low power, low voltage situations, this is not a satisfactory solution.
U.S. Pat. No. 3,075,124 teaches inserting a semiconductor device in series with the relay contacts to suppress arcing, but such an arrangement also introduces the resistance of the semiconductor device into the load current flow path, causing excessive heating of the semiconductor device. This arrangement also cannot suppress all arcing, and it is dependent upon some minimal arcing to place its arc suppression mechanism into operation.
U.S. Pat. No. 3,184,619 to Zydney and U.S. Pat. No. 4,438,472 to Woodworth both teach a suppression circuit that momentarily shorts the relay contact with a transistor switch. But because the switch is not actuated before the contacts open, there is still the possibility of contact arcing with this arrangement.
U.S. Pat. No. 4,745,511 by Kugelman shorts the contacts momentarily with a MOSFET from the moment when the relay coil is first actuated until sometime after the contacts have opened, but this arrangement requires extra power supply connections and thus cannot be built into a relay without the addition of extra power supply pins to the relay housing. This arrangement also suppresses contact closure arcing, but that can be helpful to keep the contacts clean and operative.
U.S. Pat. No. 4,959,746 by Hongel teaches using an electronic MOSFET shunting switch triggered by actuation and deactuation of the relay coil to relieve both contact closure and contact opening arcing. However, Hongel does not teach designing such a system that permits flexibility in how the relay coil is driven by external circuitry. He also does not teach how such a system can be made fail safe for use with potentially destructive high voltages relative to contact spacing. There is no xe2x80x9cfail safexe2x80x9d arrangement in Hongel""s designs.
Thus, there is a need for a simplified and improved relay contact protective circuit that can be built into a standard relay housing without the need for extra power supply connections and with minimal impact upon the flexibility of relay utilization, particularly in the case of high voltages and close contact spacing.
The present invention provides improved functionality and reduced complexity for an electronic relay switch, and it permits lower voltage relays to be used in high voltage systems. Arc suppression is provided during the xe2x80x9cbreakingxe2x80x9d of the circuit, and the arc suppression mechanism is verified before the relay is permitted to operate to avoid damage to the contacts and possible fire hazards. When an undesirable condition is detected, operation of the relay is prevented. In addition to avoiding damage to the contacts, damage to circuit components is prevented. In one embodiment, provision is made for full operability even when the relay coil connections are reversed.
Briefly summarized, the present invention may be characterized as a method for safely suppressing the arcing of a relay""s contacts when the contacts open using a solid state switch connected in parallel with the relay""s contacts as an arc suppressing device. This method comprises the steps of sensing the relay coil energization signal before its contacts close, and in response, energizing the solid state switch to short the relay contacts together. Then one tests whether the potential across the relay""s contacts drops to a very low potential in response to this energization of the solid state switch before the relay""s contacts close. If the potential across the relay""s contacts is not very low, then one prevents energization of the relay coil and subsequent coil at least until contact closure energization terminates; but if the potential across the relay""s contacts is very low, then one permits energization of the relay coil and subsequent contact closure, and one then senses deenergization of the relay before its contacts open, and in response, energizes the solid state switch again to short the relay contacts together long enough to suppress arcing that would otherwise occur when the relay""s contacts open.
The energizing of the solid state switch in response to sensing energization of the relay before its contacts close can be a brief enough energization such that some contact closure arcing is permitted to occur to keep the contacts in good condition. The method may also include the steps of permitting energization of the relay whenever its contacts are closed, and preventing energization of the relay whenever its contacts are open, but permitting energization of the relay for a brief time, long enough to permit the contact closure, after the testing described above determines the potential across the solid state switch connected in parallel with the relay""s contacts is very low. The steps of sensing energization or deenergization of the relay may be carried out by sensing a potential change at whichever end of the relay""s coil is free to change its potential in response to an incoming energization signal, such that the method works regardless of which end of the coil of the relay may be connected to a fixed potential.
The invention may also be characterized as an arc suppression system for a relay having at least one pair of normally open contacts, having an energizing coil with at least two leads, and having at least two contact terminals and two coil terminals respectively electrically coupled to each of the contacts and to each of the coil leads. This system comprises a solid state contact shorting switch having two switch leads connected in parallel with the two contact terminals and having a control lead which, when energized with an input signal, causes the switch to effectively short circuit the two contacts of the relay. It further comprises a first timing pulse generator receiving an input signal from at least one of the two coil terminals and responding to a coil energizing signal by generating a first timing pulse which, when applied to the solid state switch""s control lead, causes momentary conduction of the switch. It also comprises a circuit tester and coil switch having an input coupled to at least one of the two contact terminals independently of the switch leads of said solid state switch and arranged to signal, by its conductive or nonconductive state, whether the two relay contacts are or are not effectively short circuited by the solid state switch, this coil switch having two switch leads which form an electrically conductive part of one of the two electrical couplings between the coil leads and the coil terminals, such that the coil cannot be energized to close the relay contacts unless the coil switch first verifies the proper operation of the contact shorting switch and its connections to the contact terminals. Also, it further comprises a second timing pulse generator receiving an input signal from at least one of the two coil terminals and responding to the termination of a coil energizing signal by generating a second timing pulse which, when applied to the solid state switch""s control lead, causes conduction of the contact shorting switch of sufficient duration to suppress contact opening arcing. The first timing pulse generator may generate a timing pulse of such short duration that some contact closure arcing is permitted to occur, and the circuit tester and coil switch may include a time delay that maintains the coil switch conductive after it is first rendered conductive for sufficient time so that it remains conductive during any brief moment when the contacts are not fully closed following the opening of the contact shorting switch after the circuit testing is completed.
The two timing pulse generators may include inputs connecting to both coil terminals to sense commencement and termination of energization signals regardless of which end of the coil may not be free to fluctuate in potential.
The invention may also be characterized as a fail-safe arc suppression system for a relay whose contacts are subjected to voltages that are high relative to the relay""s maximum open contact separation distance, thereby creating a risk of possible sustained arcing if contact opening arcing is not carefully and fully suppressed. This system comprises a relay having an energizing coil having first and second coil leads respectively electrically coupled to first and second relay coil terminals of the relay and having at least first and second normally open relay contacts respectively electrically coupled to first and second relay contact terminals of the relay, the relay contacts arranged to close upon energization of the energizing coil and arranged to open in response to deenergization of the energizing coil. It also comprises a first solid state switch having a first switch control lead and having first and second switch leads, the first and second switch leads being coupled electrically respectively to the first and second relay contact terminals such that energization of the control lead causes the switch to effectively short circuit the relay contacts. In addition, it comprises a second solid state switch having a second switch control lead and having first and second switch leads that form part of the electrical coupling between at least one coil lead and its respective coil terminal such that a signal applied to the second switch control lead can permit or block the energization of the relay coil, said second switch control lead being electrically coupled to the one of said contact terminals that is intended to be connected to a load such that when the contact terminals are effectively short circuited, a signal is applied to the second switch control lead that permits the relay coil to be energized. It further comprises a circuit having at least one input electrically coupled to at least one of said first and second coil terminals and having an output electrically coupled to said first solid state switch""s control lead, with this said circuit responding to a signal change at the circuit""s input in a direction signaling the removal of energy from said energizing coil by applying a first timing signal pulse having a first pulse duration to the first solid state switch control lead, the first pulse""s duration being sufficiently long to cause the first solid state switch to effectively short circuit the relay contacts while they open to suppress contact closure arcing; and with this circuit responding to a signal change at the circuit""s input in a direction signaling the application of energy to said energizing coil by applying a pulse to the first solid state switch of sufficient duration to permit the second solid state switch to test for proper operation of said first solid state switch and its electrical coupling to the relay contact terminals before permitting the relay coil to be energized.
This system may include two timing signal pulse generators: one that responds to a signal fluctuation that indicates cessation of the coil energization signal by generating a pulse of sufficient time duration to permit full arc suppression during contact opening; and the other that responds to a signal fluctuation that indicates commencement of the coil energization signal by generating a pulse of short enough time duration to permit some contact closure arcing to occur if the circuit testing indicates that all is well and permits the relay coil to be energized to close the contacts. The circuit also may have two inputs, one from either coil terminal, such that it can always detect fluctuations of the coil energization signal regardless of which end of the coil may not be free to fluctuate in any given circuit configuration.
The invention may also be characterized as an arc suppression system for a relay having at least one pair of normally open contacts, having an energizing coil with at least two leads, and having at least two contact terminals and two coil terminals respectively electrically coupled to each of the contacts and to each of the coil leads, which also comprises a solid state contact shorting switch having two switch leads connected in parallel to the two contact terminals and having a control lead which, when energized with an input signal, causes said switch to effectively short circuit the two contacts of the relay, and a timing pulse generator having two inputs receiving input signals respectively from both of the two coil terminals and responding to a signal fluctuation at either of the coil terminals in a direction indicating a cessation of an incoming relay energizing signal by causing conduction of the contact shorting switch of sufficient duration to suppress contact opening arcing. The timing pulse generator may be designed to respond to signal fluctuations at either of the coil terminals in a direction indicating commencement of a relay energization signal by permitting at least a short duration of no arc suppression activity at contact closure time so that some amount of contact closure arcing is permitted to occur. And the timing pulse generator may also be designed to respond to signal fluctuations at either of the coil terminals in a direction indicating commencement of a relay energization signal by momentarily causing the contact shorting switch to short the relay contacts. During this brief shorting of the relay contacts, other test circuits cam be called upon to test the integrity of the contact shorting switch and the coupling of its switch leads to the contact terminals by measuring the degree to which the contact shorting switch is able to short the relay contacts, the test circuit preventing actuation of the relay if there is any doubt about the ability of the contact shorting switch to suppress contact opening arcing.
The invention further may be found in a relay that has a pair of normally-open relay contacts electrically coupled to pair of contact terminals, a relay coil magnetically coupled to at least one of the pair of relay contacts and arranged to close the pair of relay contacts when supplied with current with the coil having first and second leads, a pair of coil terminals electrically coupled to the respective coil leads, a solid state switch having a pair of switch leads electrically coupled to the respective contact terminals, the switch also having a control lead, a timing circuit having first and second signal inputs and first and second power inputs and having a signal output electrically coupled to the switch control lead with the timing circuit providing a timed output signal pulse at its signal output in response to signal transitions at either of its signal inputs, first and second electrical power couplings interconnecting the respective first and second power inputs to at least one of the coil or contact terminals with at least one of the first and second electrical power couplings interconnecting the corresponding one of the first and second power supply inputs either to both of the coil terminals or to both of the contact terminals through diodes or their equivalents, and first and second signal supply couplings interconnecting the respective first and second signal inputs to the respective coil terminals to thereby permit the timing circuit to respond to signal fluctuations on either coil terminal.
At least one of the first and second electrical power couplings may interconnect the corresponding one of the first and second power supply inputs to both of the coil terminals. And more specifically, the first electrical power coupling may interconnect the first power supply input to both of the coil terminals through diodes or their equivalents, and the second electrical power supply coupling may interconnect the second power supply input to one of the contact terminals.
Alternatively, the relay may comprise a pair of normally-open relay contacts, first and second contact terminals electrically coupled to the respective relay contacts in said pair of contacts, a relay coil magnetically coupled to at least one of the pair of relay contacts and arranged to close the pair of relay contacts when supplied with current with the coil having first and second leads, first and second coil terminals, with the first coil terminal electrically coupled to the first lead, a first solid state switch electrically coupling the second coil lead to said second coil terminal and having a first control lead, a second solid state switch having first and second switch leads and having a second control lead, first and second contact terminal electrical couplings respectively connecting the first and second switch leads to the first and second contact terminals, a timing circuit having first and second signal inputs and first and second power inputs and having a signal output electrically coupled to the second control lead with the timing circuit providing a timed output signal pulse at its signal output in response to signal transitions of a first polarity received at its first signal input and in response to signal transitions of a second polarity received at its second signal input, first and second electrical power couplings connecting the respective first and second power inputs to at least one of the coil or contact terminals, first and second signal supply couplings connecting the respective first and second signal inputs to the second coil terminal, and a circuit test electrical coupling connecting the second contact terminal to the first control lead, this circuit test electrical coupling being electrically isolated from the second contact terminal electrical coupling such that a test signal flowing from the second switch lead of the second solid state switch to the first control lead of the first switch must pass into and out of the second contact terminal.
In addition, the relay may include a printed circuit board having openings through which all of the terminals extend, and some or all of the electrical interconnections between the first and second switches and the contact and coil terminals may be formed by foil current paths formed on the surface of the printed circuit board and electrically engaging the first and second contact terminals and the second coil terminal. Also, the electrical coupling between the second coil lead and said first solid state switch may comprise a third coil terminal that is electrically coupled to the second coil lead and electrically coupled by foil on the circuit board to the first solid state switch.
The relay terminals may also include shoulders against which the printed circuit board rests, and they may include stakable portions which, when staked, lock the circuit board in between said shoulder and said stakable portion.